The research is designed to assess the relationship between the reinforcing properties of stimulant and depressant drugs and personality, mood states and subjective reactions to the drug in humans. These relationships will be assessed in a situation where subjects are given an opportunity to choose to ingest one of two available drugs with concurrent measurements of subjective effects. The effects on drug choice of manipulating a wide variety of environmental, pharmacological and organismic variables will then be determined. Variables to be evaluated include psychiatric status, age and drug history of the subject, environmental context and relative response cost of the alternatives. The purpose of these studies is to develop a method for determining the relative abuse liability of drugs and to determine whether there are certain individuals who possess characteristics which put them at greater risk for excessive drug use.